1. Field of Invention
The present invention relates to electrical heating cables.
2. Description of the Prior Art
Electrical heating cables and tapes, as exemplified by U.S. Pat. Nos. 2,719,902 and 3,757,086, have been used commercially for some time to provide heat to pipes and tanks in cold environments.
In the past, control of the temperature of these cables has been achieved by means of an external thermostat which interrupts the current flow to the cable at a specified temperature limit. These external thermostats, even when carefully installed, could be so located that the pipe or tank temperature was sensed and controlled without regard for the actual temperature of the heating cable. In addition, these thermostats were prone to failure, resulting in thermal run-away when the thermostat failed, degradation of the electrical insulation, and possible destruction of the heating cable.
Another form of heating tape was designed for use without a thermostat and used a polymer substituted for the resistance wire. The polymer was made partially conductive by compounding with small particles of carbon or other conductive material so that heat was formed when current flowed through the partially conductive polymer. However, with this form of tape, the amount of heat produced per unit length was limited. Further, at high working temperatures a loss of heat producing ability occurred with this tape.